Lithium ion cells and batteries are secondary (i.e., rechargeable) energy storage devices well known in the art. The lithium ion cell, known also as a rocking chair type lithium ion battery, typically comprises essentially a carbonaceous anode (negative electrode) that is capable of intercalating lithium ions, a lithium-retentive cathode (positive electrode) that is also capable of intercalating lithium ions, and a non-aqueous, lithium ion conducting electrolyte therebetween.
The carbon anode comprises any of the various types of carbon (e.g., graphite, coke, carbon fiber, etc.) which are capable of reversibly storing lithium species, and which are bonded to an electrochemically conductive current collector (e.g. copper foil or grid) by means of a suitable organic binder (e.g., polyvinylidene fluoride, PVdF).
The cathode comprises such materials as transition metal chalcogenides that are bonded to an electrochemically conductive current collector (e.g., aluminum foil or grid) by a suitable organic binder. Chalcogenide compounds include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese. Lithiated transition metal oxides are at present the preferred positive electrode intercalation compounds. Examples of suitable cathode materials include LiMnO2, LiCoO2, LiNiO2, and LiFePO4, their solid solutions and/or their combination with other metal oxides and dopant elements, e.g., titanium, magnesium, aluminum, boron, etc.
The electrolyte in such lithium ion cells comprises a lithium salt dissolved in a non-aqueous solvent which may be (1) completely liquid, (2) an immobilized liquid (e.g., gelled or entrapped in a polymer matrix), or (3) a pure polymer. Known polymer matrices for entrapping the electrolyte include polyacrylates, polyurethanes, polydialkylsiloxanes, polymethacrylates, polyphosphazenes, polyethers, polyvinylidene fluoride, polyolefins such as polypropylene and polyethylene, and polycarbonates, and may be polymerized in situ in the presence of the electrolyte to trap the electrolyte therein as the polymerization occurs. Known polymers for pure polymer electrolyte systems include polyethylene oxide (PEO), polymethylene-polyethylene oxide (MPEO), or polyphosphazenes (PPE). Known lithium salts for this purpose include, for example, LiPF6, LiClO4, LiSCN, LiAlCl4, LiBF4, LiN(CF3SO2)2, LiCF3SO3 , LiC(SO2CF3)3, LiO3SCF2CF3, LiC6F5SO3, LiO2CF3, LiAsF6, and LiSbF6. Known organic solvents for the lithium salts include, for example, alkylcarbonates (e.g., propylene carbonate, ethylene carbonate), dialkyl carbonates, cyclic ethers, cyclic esters, glymes, lactones, formates, esters, sulfones, nitrites, and oxazolidinones. The electrolyte is incorporated into pores in a separator layer between the cathode and anode. The separator may be glass mat, for example, containing a small percentage of a polymeric material, or may be any other suitable ceramic or ceramic/polymer material. Silica is a typical main component of the separator layer.
Lithium-ion cells made from pure polymer electrolytes, or liquid electrolytes entrapped in a polymer matrix, are known in the art as “lithium-ion polymer” cells, and the electrolytes therefore are known as polymeric electrolytes. Lithium-polymer cells are often made by adhering, e.g. by laminating, thin films of the anode, cathode and electrolyte together wherein the electrolyte/separator layer is sandwiched between the anode and cathode layers to form an individual cell, and a plurality of such cells are bundled together to form a higher energy/voltage battery.
In lithium ion polymer cells, the same polymer is used to adhere the current collector to the electrodes, and to adhere the separator to the electrodes. Use of a copolymer beneficially yields good adhesion of the electrodes to the separator, which is required for optimum ion transfer and cell performance, but reduces any possibility of high-temperature drying of the electrodes, which beneficially removes water. This is because the copolymer has a lower melting point relative to a homopolymer, and thus cannot withstand high-temperature drying as would a homopolymer. Use of a homopolymer beneficially allows for high temperature drying of the electrodes and less swelling within the electrode and separator, desirably retaining the porous structure and thereby allowing electrolyte flow within the entire electrode, but yields reduced adhesion of the electrodes to the separator.
There is thus a need to develop methods to assemble and produce a lithium cell with polymers that allow high-temperature drying of the electrodes, and with less swelling of the polymer and less resulting loss of electrical conductivity in the electrode, and that also yield good adhesion of the electrodes to the separator.